This invention relates to a highly oriented molded article of ultrahigh-molecular-weight polyethylene and a process for producing the molded article. More specifically, this invention relates to a process for producing a highly oriented molded article of ultrahigh-molecular-weight polyethylene by the solution molding technique which comprises the steps of preparing a high-concentration molding solution of ultrahigh-molecular-weight polyethylene, extruding the solution from nozzles, taking up the resulting extrudate while applying a draft to it, and crystallizing the ultrahigh-molecular-weight polyethylene in the extrudate; and to a highly oriented molded article so produced.
Ultrahigh-molecular-weight polyethylene has superior impact strength, abrasion resistance, chemical resistance and tensile strength to general-purpose polyethylenes, and has been gaining increasing acceptance as engineering plastics. It is difficult however to fabricate it by extrusion or injection molding because it has a much higher melt viscosity, and therefore lower flowability, than the general purpose polyethylenes. In most cases, the high-molecular-weight polyethylene is fabricated by compression molding, and articles of limited forms, such as rods, are presently produced from it by extrusion at low speeds.
Japanese Patent Publication No. 9765/1962 proposed a process for producing monofilaments of high-density polyethylene by drawing at a high ratio, which comprises incorporating a high boiling additive having a higher melting point than polyethylene in an amount of 20 to 150% in the polyethylene, preparing a high-concentration dispersion of the mixture, forming a primary fibrous material from it, and hot-drawing the fibrous material to 3 to 15 times the original length while leaving 5 to 25% of the additive in it. This patent document, however, only discloses general-purpose high-density polyethylene having an MFR of 0.7 to 5.0 g/10 min. The draw ratio used is 3.4 to 11.6, and the drawn filaments have a tensile strength of 3.5 to 13.6 g/d. The patent document gives a description on the effect of drafting during spinning, but the experimental results shown there indicate that as the draft, i.e. the spinning speed, increases, the maximum draw ratio decreases, and with it, the tensile strength of the product decreases. It is seen therefore that with polyethylene having such a degree of polymerization corresponding to an MFR of 0.7 to 5.0 g/10 min., an increase in draft does not contribute to improved tensile strength.
U. S. Pat. Nos. 4,422,993 and 4,430,383 proposed a process which comprises spinning a solution of linear polyethylene having a molecular weight of at least 400,000 which is higher than the molecular weights of general-purpose polyethylenes, and drawing the spun filaments at such a temperature that the drawn filaments have a modulus of at least 20 GPa. This process enables the production of monofilaments having higher tenacity and modulus of elasticity than drawn filaments of the general-purpose high-density polyethylene. These patents describe that the polymer concentration of the solution of high-molecular-weight polyethylene is 1 to 50% by weight. However, in the actual examples disclosed in the patents, the concentration is 8% by weight at the highest. In fact, it is extremely difficult to prepare a uniform high-concentration solution of such ultrahigh-molecular-weight polyethylene. An ordinary method involving simply heating the polymer and solvent together cannot give such a solution, and a special method must be employed (see European Laid-Open Patent Publication No. 135253).
It is considered that an undrawn product (dry gel fiber) obtained from a solution having such a relatively low polymer concentration is of a lamellar porous structure [see Kalb and Pennings, Polymer Bulletin, vol. 1, 878-80 (1979), Polymer, 2584-90 (1980), and Smook et al., Polymer Bulletin, vol. 2, 775-83 (1980).
British Patent No. 1,100,497 proposes a process which comprises dissolving a linear or branched, non-crosslinked polymer in a non-polymerizable compound in a concentration of 2 to 33% by weight, extruding the solution downwardly from a spinneret into non-heated air to cool the extrudate by the non-heated air and lower its temperature, thereby separating the polymer from the solution and forming filaments, and winding up the filaments without evaporating the solvent. Example 21 of this document discloses the spinning of a 3% naphthalene solution of polyethylene ACX (a product of Allied Chemical Corp., U.S.A. which it is said has a molecular weight between 1,000,000 and 3,000,000). As in the two U.S. Patents cited above, this patent document also fails to disclose an example of using a high-concentration solution of ultrahigh-molecular-weight polyethylene.
It has been found that according to the solution-spinning method using a low concentration solution of ultrahigh-molecular-weight polyethylene, when filaments obtained by extruding the solution from a spinneret are placed under a draft, the tensile force generated by the draft cannot be transmitted fully to the polyethylene molecules because there are few points of entanglement between molecular chains of the polymer in the solution; and that consequently, an increase in density owing to molecular orientation cannot be achieved, and the drafting itself cannot contribute to the increasing of the tenacity of the fibers.
In any case, since the solution has a low polymer concentration, even when the filaments are wound up at high speeds under the application of a draft, the yield of the final product is low, and the amount of the solvent to be recovered is too large.
European Laid-Open Patent Publication No. 64167 proposes a process which is an improvement over the process disclosed in the two U.S. Patents cited above. This process comprises dissolving polyethylene in a first non-volatile solvent, spinning the solution, extracting the first solvent from the filaments by using a second volatile solvent, drawing the filaments before or after the extraction, and thereafter, removing the second solvent by drying. The patent document states that the concentration of the polymer in the first solvent is 2 to 15% by weight, preferably 4 to 10% by weight. Hence, like the above-mentioned processes, this process also uses a spinning solution having a relatively low polymer concentration. The document further states that since the application of a draft at the time of extruding the first solvent solution seems to be very deleterious on the various properties of the final filaments, the draft ratio is preferably below 2:1 and drafting should be avoided as much as possible (page 8, lines 36-38 of European Laid-Open Patent Publication No. 64167). The patent document discloses that the structural requirements which the gel filaments (xerogel) with very little porosity obtained by such a process are that they should be unoriented undrawn filaments having a crystalline orientation function, measured by broad-angle X-ray diffractometry, of less than 0.2, preferably less than 0.1, a crystallinity index, measured by broad-angle X-ray diffraction, of less than 80%, preferably less than 75 and the size of spherulites crossing the diameters of the filaments has a partial deviation of less than 0.25.
All of the processes discussed above only show the use of spinning solutions having a relatively low polymer concentration in the solution spinning of ultra-high-molecular-weight polyethylene. The following statement of Dr. Paul Smith, an inventor of the above-cited U.S. Pat. No. 4,422,993 may be partly useful in considering a reason for this state of the art. Dr. Smith stated in an argument submitted during the examination of Japanese Laid-Open Patent Publication No. 15408/1981 corresponding to the above U.S. Patent that to obtain drawn filaments having high tenacity, gel-like filaments containing a solvent having a polymer concentration of generally less than 25% by weight, preferably less than 10% by weight, should be cooled without substantially evaporating the solvent. In other words, Dr. Smith stated that for the production of filaments of high tenacity, crystallization should be performed in such low polymer concentrations as to prevent the increase of the concentration of the spinning solution attributed to the evaporation of the solvent.
Mitsuhashi et al. (Papers for 1984 Meeting of Research Institute for Polymeric Materials for Fibers, pages 107-113) state a gel film obtained by crystallization of a solution having a concentration of 15% or less assumes a long period structure of about 100 .ANG. as a result of observing its X-ray small-angle scattering, and that the dry gel film evidently has a porous structure based on a lamellar structure as determined by observation under an electron microscope. It was also determined by a similar method of observation that the dry gel film obtained from the solution having a concentration of at least 15% by weight has a long period structure of about 200 .ANG. and a dense spherulitic structure. The article concludes that as the concentration of polyethylene increases, the lamellar structure changes to spherulites in which the molecular chains (tie molecules) get into spaces between the crystalline lamellae. The paper goes on to state that a dry gel film having a few number of entangled molecular chains (tie molecules) obtained by crystallization from a solution having a concentration of 15% by weight or less shows good drawability, whereas a dry gel film obtained by crystallization from a high-concentration solution has a large number of such tie molecules, and as a result of abrupt increase of the stress in the drawing step, the draw ratio to be reached is lowered and the film fractures, with the result that the properties of the product become saturated early.
As stated above, the prior established theory was that fibers of high modulus and strength cannot be obtained from high-concentration solutions.
Later, a process for producing a high-concentration solution of ultrahigh-molecular-weight polyethylene by subjecting the polyethylene and a solvent to a swelling treatment at a specific temperature was proposed (see European Laid-Open Patent Publication No. 135,253). It has been found however that even when a high-concentration solution of ultrahigh-molecular-weight polyethylene obtained by this process is simply spun by the process disclosed in U.S. Pat. No. 4,422,993 disclosed above, it is difficult to obtain drawn filaments having high strength and modulus.
U.S. Pat. No. 4,545,950 discloses a process which comprises melt-kneading ultrahigh-molecular-weight polyethylene with a paraffinic wax having a melting point, determined by the DSC method, of 40.degree. to 120.degree. C. at a temperature of 180.degree. to 280.degree. C., extruding the mixture from a spinning die under a draft at a draft ratio of not less than 2, solidifying the resulting unstretched extrudate, and stretching it at a stretch ratio of at least 3. According to this process, the extrudate from the spinning die is solidified before it is stretched. Hence, not only the polyethylene but also the paraffinic wax in it is solidified.
It is an object of this invention to provide a process for producing a stretched molded article of ultrahigh-molecular-weight polyethylene which has high modulus and strength from a concentrated solution of the polyethylene.
Another object of this invention is to provide a process for producing a stretched molded article of ultrahigh-molecular weight polyethylene which comprises the steps of extruding a concentrated solution of the polyethylene from a die and taking the extrudate while positively applying a draft to it in spite of the fact that previously, the application of a draft has been considered undesirable.
Still another object of this invention is to provide pre-oriented molded article (unstretched molded article) which has good creep properties and is suitable for giving stretched molded articles of ultrahigh-molecular-weight polyethylene having high modulus and strength; and a process for producing such stretched molded articles.
Further objects of this invention along with its advantages will become apparent from the following description.